The ideal recording material for holography should have a spectral sensitivity well matched to available laser wavelengths, a linear transfer characteristic, high resolution, and low noise, be indefinitely recyclable or relatively inexpensive. While several materials have been studied, none has been found that meets all these requirements. The lack of available materials for phase holograms has stimulated a search for new approaches.
The new approach was described in the co-pending U.S. patent application Ser. No. 10/665,339 filed on Sep. 19, 2003, which teaches how a photo-thermal process based on precipitation of dielectric microcrystals in the bulk of glass exposed to UV radiation can be used to record a high-efficiency volume phase hologram in glass because of a difference between refractive indices of exposed and unexposed areas of glass blank.
According to co-pending '339 application and references cited herein, the first step of the proposed process is the exposure of the glass sample to UV radiation, which produces ionization of a cerium ion. The electrons released from cerium are then trapped by a silver ion. As a result, silver is converted from a positive ion to a neutral atom. This stage corresponds to a latent image formation and no significant coloration or refractive index change occurs. The next step is thermal development.
This development process includes two stages. The first involves the high diffusion rate silver atoms possess in silicate glasses. This diffusion leads to creation of tiny silver containing particles at relatively low temperatures with a range of approximately 450 to approximately 500° C. A number of silver clusters arise in exposed regions of glass after aging at elevated temperatures. These silver particles serve as the nucleation centers for sodium and fluorine ion precipitation and cubic sodium fluoride crystal growth occurs at temperatures between 500° C. and 550° C. Interaction of crystalline phase with glass matrix at elevated temperatures results in decreasing of refractive index in exposed areas compare to that in unexposed ones. This phenomenon was named the “photo-thermo-refractive” (PTR) process. Glasses, which possess such properties, were called “photo-thermo-refractive” (PTR) glasses.
Conditions of glass technology, exposure, and development were found in that work to create volume holographic gratings referred to as Bragg gratings, with relative diffraction efficiency up to approximately 97%. The maximum recorded spatial frequency was about 10,000 mm−1 and the gratings are stable up to approximately 400° C. The photosensitivity was found in the range of several hundred mJ/cm2 at a helium-cadmium laser wavelength of approximately 325 nm. The absorption band of Ce3+, which is used for photo-ionization, has a maximum wavelength at approximately 300 nm and a long wavelength tale up to approximately 350 nm. This means that several commercial lasers such as N2, Ar, He—Cd, etc., can be used for recording. Once developed, holograms in PTR glass are not destroyed by further exposure to UV, IR or visible light. These properties of PTR holographic elements resulted in wide application of this technology for different laser systems operating in visible and near IR spectral regions.
The most important applications of holographic optical elements require nonplanar holograms working in visible and near IR regions. Such elements, if used as selective focusing, defocusing, correlating, etc. components, can dramatically decrease sizes, weight, and cost of laser and optical systems. However, nonplanar holograms can only be reconstructed by the same wavelength that was used in the recording process. This means that by linear photosensitivity, complex PTR holograms can only be done in the UV region within the absorption band of cerium which is placed in the short wavelength region less than 350 nm.
The co-pending '339 parent patent application teaches how to extend sensitivity of PTR glass to the visible region by the use of two-step exposure to low power UV radiation followed by high power visible radiation. This method teaches generation of latent images by a conventional continuous wave UV irradiation followed by nonlinear bleaching of this latent image by high power visible radiation. However, no sensitivity was observed for longer wavelengths.
At longer wavelengths of illumination, PTR glass is transparent and shows no linear photosensitivity. In contrast, exposure to high intensity laser pulses at wavelengths longer than the linear photosensitivity edge of PTR glass can excite PTR glass by nonlinear mechanisms. It is shown in O. M. Efimov, L. B. Glebov, S. Grantham, M. Richardson, Photoionization of silicate glasses exposed to IR femtosecond pulses, Journal of Non-Crystalline Solids, 253, (1999) pp. 58-67 that exposure of PTR glass to high intensity infrared femtosecond pulses causes photoionization and subsequent refractive index change after thermal development. The co-authors of the publication noticed that refractive index change occurred by observation of microscope photographs of channels formed inside bulk PTR glass. However, no real attention was given to this phenomenon. First of all, no characterization of the amount of refractive index change was carried out. In addition, the co-authors did not investigate the sign of refractive index change, i.e. if it appears as refractive index decrement due to the photo-thermo-refractive process or refractive index increment due to glass densification.
Because the nonlinear processes involved in exciting PTR glass under femtosecond exposure produces photoionization of the glass matrix directly, it is unnecessary for certain dopants to be included in the standard PTR glass composition. In Y. Kondo, K. Miura, T. Suzuki, H. Inouye, T. Mitsuyu, K. Hirao, Three-dimensional arrays of crystallites within glass by using non-resonant femtosecond pulses, Journal of Non-Crystalline Solids, 253, (1999) pp. 143-156 it was shown that a photosensitive glass similar to PTR glass but without cerium can be photoionized by exposure to IR femtosecond pulses and crystallized after thermal development. But because the precipitated crystals were large in size (about 8 μm in diameter) this glass had high scattering and absorption and therefore could not be used for the recording of high efficiency refractive or diffractive elements. Furthermore, the co-authors tried to ionize their PTR-like glass with nanosecond pulses but were unsuccessful. Photosensitivity of PTR glass to nanosecond pulses has not been previously reported. However photoionization of high purity alkali-silicate glasses by UV nanosecond pulses is described in Gagarin, A. P., Glebov, L. B., Efimov, O. M., and Efimova, O. S., Formation of color centers in sodium calcium silicate glasses with the nonlinear absorption of powerful UV radiation, Sov. J. Glass Phys. Chem., 5, pp. 337-340 (1979).